1. Field of the Invention
This invention relates to a process of concentrating dilute phosphoric acid, which is conducted in a plurality of separate phosphoric acid cycles, which are associated with respective vacuum evaporator stages connected in series, and is indirectly heated in each cycle outside the evaporators by means of surplus heat from a contact process sulfuric acid plant.
2. Discussion of the Prior Art
The treatment of raw phosphates with sulfuric acid often results in a dilute phosphoric acid which contains about 26 to 32% P.sub.2 O.sub.5 which for use in the production of fertilizer, must be concentrated to a concentration of more than about 45% P.sub.2 O.sub.5.
From A.V. Slack "Phosphoric Acid", Vol. II, 1968, published by Marcel Dekker, Inc., New York, pages 607 to 634, it is known to concentrate phosphoric acid in a process in which the phosphoric acid is conducted in a plurality of separate phosphoric acid cycles, which are associated with respective vacuum evaporator stages connected in series, and is indirectly heated in each cycle outside the evaporators by means of low-pressure steam, which is produced by means of surplus heat from a sulfuric acid process contact process. Because much energy is consumed in concentrating phosphoric acid, a large proportion of the high-grade energy produced in the form of steam in the sulfuric acid contact process plant is spent for such concentrating. Unless it was required in the form of valuable steam for concentrating the phosphoric acid, the energy supplied from the sulfuric acid contact process plant could be used to produce more-electric power to meet the total energy requirement of the combined plant for producing sulfuric acid and phosphoric acid.
It is an object of the invention to provide a process for concentrating phosphoric acid in a combined plant for producing sulfuric acid and phosphoric acid which avoids, as far as possible, the use of high-grade energy for concentrating the dilute phosphoric acid and provides a substitute for such energy in a manner which is technologically simple and economical.